1. Field of the Invention
This invention relates to an optical pickup device for recording/reproducing information signals on or from an optical recording medium by radiating a light beam thereto from a light source, and a focusing servo device for controlling the focusing position along the optical axis of the optical pickup device.
2. Description of the Related Art
There has hitherto been known a magneto-optical or a phase change type optical recording medium as an optical recording medium for recording and/or reproducing information signals by means of a light beam from a laser light source radiated thereto. The magneto-optical recording medium, as an example, is comprised of a transparent substrate or a light-transmitting substrate, formed of, for example, a light transmitting synthetic resin, such as polycarbonate. The magneto-optical medium further comprises a magnetic layer, a dielectric layer and a surface protective layer, stacked on a major surface of the substrate. The magnetic layer, which may be a rare earth--transition metal alloy thin film, has an axis of magnetization perpendicular to the film surface and exhibits superior photomagnetic effects. Information signals may be recorded or reproduced by radiating a light beam, such as a laser beam, from the transparent substrate side.
In such optical recording medium, it is desirable to improve the recording density further in view of the fact that digital video signals can require upwards of ten times the data volume of digital audio signals. Furthermore, there is a demand for smaller size recording media. For instance, there is a demand for smaller digital audio disc players, which would require smaller discs, and concomitantly, higher density of data storage. Similarly, for recording general computer data, there is an increasing demand for a smaller sizes and larger recording capacities of the recording medium, in order to accommodate increasingly popular notebook or diary size computer devices.
The information recording density on the recording medium is governed by the wavelength .lambda. of the laser light and the numerical aperture NA of an object lens, so that when increasing the recording density, it is necessary to decrease the wavelength .lambda. of the laser light and/or for the numerical aperture NA to be increased. One widely used system for high density reproduction is the co-focal point detection optical system. In this system a pinhole is provided at a focal point of a lens at which the return light from the disc is converges. This convergence of light is detected by a light receiving or detecting element.
The focusing servo devices in such high density optical disc systems have a deficiency in that the defocusing tolerance becomes extremely narrow. For example, an optical pickup device making use of a light source of a short wavelength laser light, such as green or blue light, or an object lens having a larger NA value, has a smaller or shallower depth of focus. On the other hand, with an optical pickup device employing the so-called co-focal point detection system, the light volume of the detected light undergoes extremely large changes with defocusing due to its sharp depth response characteristics.
The conventional focusing servo device shifts the object lens along the optical axis, i.e. in the focusing direction, by means of an electromagnetic coil. It is difficult with the conventional servo device to inhibit focusing fluctuations with respect to resolution and bandwidth due to high frequency disturbances produced within the depth of focus, such as minute fluttering of the rotating discs. There is, therefore, a need for a device for accurately detecting focal point error signals within the depth of focus.
Focusing fluctuation problems are compounded by the fact that the optical disc is subject not only to warping in the radial direction but to undulations in the circumferential direction. Furthermore, the optical disc is occasionally loaded in a tilted state on the recording/reproducing apparatus. That is, in the recording/reproducing apparatus, the light spot on the information recording surface tends to be distorted not only in the radial direction but also along the recording track(s).
If the spot shape is changed so that its long axis direction coincides with the recording track direction, inter-symbol interference is produced in the reproduced signals in the recording/reproducing apparatus, so that, if the pits are close to one another, the recording signals cannot be reproduced accurately. Conversely, if the long axis direction is coincident with the radial direction of the optical disc, crosstalk is produced between the recording tracks which renders it impossible to record the information correctly when the track pitch is narrow.
This means that if the recording density is to be improved in the conventional recording and/or reproducing apparatus, it becomes necessary with the conventional recording/reproducing apparatus to correct for the tilt of the optical disc promptly to compensate for such changes in the optical disc orientation.
However, since these changes in the optical disc are composed of components changed with disc rotation at a frequency from tens of cycles per second (Hz), to upwards of one hundred Hz, it is not possible with the conventional system of correcting the position of the disc to entirely correct for these changes in the optical disc.
Recording density might also be improved by decreasing the tilt of the optical disc and thereby decreasing orientation changes during playback. However, even granting that the tilt of the optical disc may be tolerated up to 0.6.degree. by employing an object lens having a numerical aperture of 0.45, the tolerable range of the tilt is decreased to 0.25.degree. with the use of an object lens having the numerical aperture of 0.6. This is because the coma aberration is changed in proportion to the numerical aperture of the object lens cubed. Consequently, an object lens having a higher numerical aperture required for improving the recording density cannot be employed. Difficulties are met in the application of the above-described methods of adapting for disc orientation changes in order to improve the recording density.